The present invention relates to a method for operating an exhaust gas treatment system, comprising an SCR catalytic converter for treatment an exhaust gas of a motor vehicle internal combustion engine, wherein a reducing agent containing ammonia is fed to the exhaust gas and an exhaust gas enriched with NH3 corresponding to the dosing is fed to the SCR catalytic converter, and an ammonia filling level value for a filling state of ammonia stored in the SCR catalytic converter is calculated by a computer model.
For the catalytically supported removal of nitrogen oxides (NOx) from internal combustion engine exhaust gases, it is known to add aqueous urea solution as a reducing agent containing ammonia (NH3) to the exhaust gas. In the hot exhaust gas, NH3 is released as the actual selective reducing agent with regard to the NOx reduction at a so-called SCR catalytic converter by means of thermolysis and/or hydrolysis. Thus, one problem is determining an appropriate feed rate for the reducing agent while avoiding an overdosing which can result in an undesired slip of NH3.
For solving these problems, EP 1 348 477 A1 discloses determining a usage rate of NH3 stored in the catalytic converter and a NOx treatment rate and to control the reducing agent feed in dependence on these variables in connection with a filling level of NH3 stored in the catalytic converter.
Published application EP 0 554 766 A1 discloses a method for the NOx reduction in exhaust gases by controlled hyperstoichiometric addition of NH3, wherein NH3 is dosed upstream of a catalytic converter until the NH3 amount stored in the catalytic converter has reached an upper threshold value. The NH3 stored in the catalytic converter is used with the NOx emitted by the internal combustion engine and fed to the catalytic converter, whereby the toxic substance NOX is removed from the exhaust gas. The NH3 feed is again carried on when the NH3 amount stored in the catalytic converter has reached a lower threshold.
In these and other known similar methods, a model-based combination of measurement values and stored characteristic values often takes place, for example for the SCR catalytic converter. A NH3 feed rate and a modeled usage rate are balanced by an integration continuously carried out and the reducing agent dosing rate is chosen in such a manner that a desired NH3 filling level in the catalytic converter and a catalytic converter efficiency. It has, however, been shown that errors caused by, for example, parameter changes add up in the course of time, so that the correspondence between the modeled NH3 filling level of the catalytic converter and the actual value worsens and dosing errors result in the form of an underdosing or overdosing. The result of this is a reduced treatment effect or an increased NH3 slip. As a solution for this problem, it is suggested in DE 101 00 420 A1, which adapts the dosing rate in that at least one relevant measuring variable is sensed with certain operating states and is adapted to an expected value by correction. It has however been shown that further improvements are necessary to be able to meet requirements for the highest nitrogen oxide conversions.
Accordingly, exemplary embodiments of the present invention provide a method for operating an exhaust gas treatment system with an SCR catalytic converter, which enables a nitrogen conversion that is improved further.
An exemplary method for operating an exhaust gas treatment system, comprising an SCR catalytic converter for the treatment of an exhaust gas of a motor vehicle internal combustion engine is provided. The method involves adding, by a dosing unit controllable by a control unit, an ammonia-containing reducing agent to the exhaust gas; feeding an exhaust gas enriched with ammonia according to the dosing to the SCR catalytic converter; calculating, by a computer model, an ammonia filling level value for a filling state of ammonia stored in the SCR catalytic converter; and calculating, by the computer model, a model dosing rate for the dosing of the reducing agent into the exhaust gas at which a target filling level of ammonia stored in the SCR catalytic converter specified by the computer model or a target efficiency specified by the computer model for a nitrogen oxide conversion with ammonia stored in the SCR catalytic converter or fed to the SCR catalytic converter are at least approximately achieved. A correction by a changeable long term adaption factor to a target dosing rate is provided for the model dosing rate, a correction by a changeable short term adaption factor to an assumed actual filling level is provided for the ammonia filling level value, the dosing unit is accessed for emitting the target dosing rate, integrations in successive integration cycles of a presettable length respectively determine a nitrogen raw emission value, a nitrogen pure emission estimated value and a nitrogen pure emission measuring value and further a difference of the nitrogen pure emission measuring value and the nitrogen pure emission estimated value related to the nitrogen oxide raw emission value as the adaption characteristic value, and the adaption characteristic value is used for assessment of a long term or short term adaption.
With the method according to the invention, a NH3 filling level value for a filling level of ammonia stored in the SCR catalytic converter is calculated and a model dosing rate for the dosing of the reducing agent into the exhaust gas is calculated by the computer model, with which a target filling level of NH3 stored in the SCR catalytic converter predetermined by the computer model or a predetermined target efficiency for a NOx conversion with NH3 stored in the SCR catalytic converter and/or fed to the catalytic converter shall be reached at least approximately. In a further arrangement of the invention, a correction to a target dosing rate by a changeable long term adaption factor is provided for the model dosing rate with the active filling level regulation and/or with an active efficiency control and in a The dosing unit is correspondingly accessed for emitting the target dosing rate.
Thus, the present invention provides the ability to perform a correction acting directly on the model dosing rate, which will become immediately effective, but is possibly maintained over a longer time. On the other hand, the possibility is provided to perform a correction of the modeled NH3 filling level acting in the short term. In this manner, compensation can be performed for disturbing influences occurring in the short term and disturbing influences effective over a longer time, such as gradually progressing ageing or drift effects.
In an arrangement of the invention NOx and/or NH3 emission values of the exhaust gas are determined continuously and the NOx and/or NH3 emission values are evaluated in a cyclic manner by the control unit with regard to fulfilling a predetermined short term adaption criterion and/or a long term adaption criterion. With this advantageous arrangement, a plausibility check of the modeled NH3 filling level value takes place in successive cycles by adjustment with emission values. The emission values can be determined in a measurement-technical manner or be taken from operating characteristic fields. A modeled NH3 filling level value, possibly corrected subsequent to a cycle to the assumed actual filling level, then serves as the current filling level value, on which a further balancing integration and dosing sits in order to achieve the target filling level or the target efficiency. This is synonymous with a short term adaption of the NH3 filling level value, with which is reacted promptly to short term influences, which possibly lead to a false modeling of the NH3 filling level. However, a correction of the model dosing rate possibly performed particularly by multiplication with the long term adaption factor has an effect beyond a cycle, which is synonymous with a long term adaption. By the procedure according to the invention with a cyclically performed check of modeled operating variables, a cyclic adaptation of these operating variables is enabled, if this is necessary. This ensures a highly exact modeling, and an optimal NH3 filling level adjustment and optimal catalytic converter efficiency are achieved.
In a further arrangement of the invention, by integration in successive integration cycles of a presettable length, respectively a NOx raw emission value, a NOx pure emission estimated value and a NOx pure emission measuring value and further a difference of the NOx pure emission measuring value and the NOx pure emission estimated value related to the NOx raw emission value as the adaption characteristic value (K) are respectively determined in parallel.
The NOx raw emission value is preferably determined by integration of an NOx content in the exhaust gas on the input side of the SCR catalytic converter. The NOx content can be obtained on the input side of the SCR catalytic converter in a measure-technical manner by a correspondingly arranged NOx sensor or in a model-based manner. The NOx raw emission value represents a NOx amount flowing into the SCR catalytic converter in a respective integration cycle and thus the NOx load of the SCR catalytic converter.
The estimated NOx raw emission value can be determined by integration of an NOx content in the exhaust gas on the output side of the SCR catalytic converter calculated by the computer model. The computer model estimates the NOx conversion rate of the catalytic converter using present operating variables such as model dosing rate, actual filling level, catalytic converter temperature, exhaust gas mass flow and possibly further variables. By integration over an integration cycle, an NOx amount as NOx pure emission estimate value leaving the SCR catalytic converter in the integration cycle results together with the NOx amount flowing into the SCR catalytic converter.
The NOx raw emission measuring value can be determined by integration of an NOx content in the exhaust gas on the output side of the SCR catalytic converter derived from a signal of a NOx sensor arranged on the output side of the SCR catalytic converter.
By forming the difference of the NOx pure emission estimated value and the NOx pure emission measuring value, deviations of the modeled NOx pure emission are sensed from the measured pure emission. The adaption characteristic value thereby forms a very informative characteristic value with regard to the modeling quality of the computer model as the difference referred to the NOx raw emission value.
The adaption characteristic value is consulted according to the invention as the decisive characteristic value for the assessment of the necessity of a long term and/or short term adaption. In particular, the long term criterion is viewed as fulfilled in a further arrangement of the invention, if the adaption characteristic value exceeds a presettable threshold and in a still further arrangement of the invention the short term adaption criterion is viewed as fulfilled if the adaption characteristic value falls below a presettable lower threshold.
In a further arrangement of the invention, the long term adaption factor is changed by a presettable amount with a fulfilled long term criterion and the short term adaption factor is changed by a presettable amount with a fulfilled short term adaption criterion. The amount by which the long term or the short term is possibly changed, can be provided in a fixed manner or be preset depending on the magnitude of the adaption characteristic value. In the later case, it can be corrected in a correspondingly high manner analogously to a proportional regulation with comparatively high error modelings, whereby an approach to the target variables is enabled in a correspondingly fast manner.
In a further arrangement, a current filling capacity for a NH3 amount that can currently maximally be stored in the SCR catalytic converter and a currently maximally possible efficiency for a NOx conversion of the SCR catalytic converter are determined, and, when falling below a presettable threshold for the current filling capacity, are preset as the target efficiency. For example, with a high catalytic converter temperature, the filling capacity of the SCR catalytic converter can be so low that the adjustment of the target filling level is difficult. Using the procedure according to the invention, an exact modeling is also enabled in these cases, as the target dosing rate of the reducing agent is then adjusted with a view to the target efficiency of the catalytic converter. According to the invention, an adaption is also provided with this operating manner. For this, a long term adaption factor is fixed in such a manner in a further arrangement of the invention that the resulting NOx raw emission estimated value would have led to an adaption characteristic value of zero or approximately zero in the previous integration cycle.
Advantageous embodiments of the invention are illustrated in the drawings and are described in the following. The previously mentioned characteristics and which will still be explained in the following cannot only be used in the respectively given combination but also in other combinations or on their own without leaving the scope of the present invention.